The present invention relates to a direction finder for a vehicle, in which a moving direction is detected by detecting terrestrial magnetism.
FIGS. 1 and 2 will be used to describe the principle of a direction finding operation of a conventional direction finder for use in a vehicle. A horizontal component H (hereinafter referred to as "terrestrial magnetism H") of terrestrial magnetism is detected by a terrestrial magnetism detector 2 mounted on a vehicle 1 as shown in FIG. 1. Assuming that a moving direction A of the vehicle 1 makes an angle .theta. with respect to a direction of the geographic magnetism H, i.e., north, the terrestrial magnetism detector 2 detects a component H.sub.y (=H cos .theta.) of the terrestrial magnetism H in the same direction as the vehicle moving direction, and a component H.sub.x (=H sin .theta.) orthogonal to the moving direction A. These components are converted by the terrestrial magnetism detector 2 into electric signals, respectively, which are then amplified to provide a pair of detection signals x and y according to the following equations EQU x=KH.sub.x =KH sin .theta. (1a) EQU y=KH.sub.y =KH cos .theta. (1b)
where K is a magnetism-to-voltage conversion constant. The signals x and y may be d.c. voltages. As evident from the equations 1a and 1b, to provide detector calibrator the detected signals x and y should be suitably calibrated to zero when the field components H.sub.x and H.sub.y are zero so that the magnitudes of the signals x and y are proportion to intensities of the components H.sub.x and H.sub.y, respectively, and can be used as reference values.
FIG. 2 is an x-y perpendicular coordinate system on which points, each being defined by magnitudes of the electric signals x and y, are plotted. A locus of the plot describes a circle O.sub.1 and an angle .theta., i.e., the orientation of the vehicle 1 is given by an equation: EQU .theta.=tan.sup.-1 (x/y) (2)
Due to limitations in an initial calibration operation and due to slight errors often induced by in avoidable errors inherent in an operation of a detector (e.g., error per distance or area traveled), in reality there exists a potential that the direction of the terrestrial magnetism H is not coincident with the geographical north and there is an error, i.e., declination therebetween. For simplicity and ease of discussion it is assumed here that there is no such error.
As another source of error, it has been known that, due to magnetization of magnetic material of various components constituting the vehicle, an orientation .theta. calculated according to the equation (2) is not always correct.
Describing this in more detail with reference to FIGS. 3 and 4, the vehicle 1 is subjected to a magnetic field H.sub.v shown in FIG. 3 produced by those magnetized components. With the magnetic field H.sub.v, the magnetic field to be detected by the terrestrial magnetism sensor 2 becomes a magnetic field H.sub.e which is a composite of the terrestrial magnetism H and the magnetic field H.sub.v. Coordinates (x,y), (x.sub.v, y.sub.v) and (x.sub.e, y.sub.e) of signals from the sensor 2 which correspond to coordinates (H.sub.x,H.sub.y), (H.sub.vx,H.sub.vy) and (H.sub.ex,H.sub.ey) are shown in FIG. 4. Thus, the signal x.sub.e and y.sub.e from the sensor 2 are represented by EQU x.sub.e =x+x.sub.v =K.sub.1 H sin .theta.+x.sub.v ( 3a) EQU y.sub.e =y+y.sub.v =K.sub.2 H cos .theta.+y.sub.v ( 3b)
where the angle .theta..sub.e obtained from the signals x.sub.e and y.sub.e according to the equation (2) becomes EQU .theta..sub.e =tan.sup.-1 (x.sub.e /y.sub.e) (4)
Thus, a true orientation .theta. can not be obtained.
However, since the field H.sub.v is produced by the vehicle 1 acting as a permanent magnet and an intensity and direction thereof with respect to the moving direction A of the vehicle 1 are constant, the coordinates (x.sub.v, y.sub.v) of the signal corresponding to the magnetic field H.sub.v shown in FIG. 4 is kept unchanged even if the direction A is changed. Therefore, a locus of the coordinates (x.sub.e,y.sub.e) of a detection signal when the vehicle 1 travels along a circle route becomes an ellipse O.sub.3 having a center (x.sub.v, y.sub.v) and an eccentricity K.sub.2 /K.sub.1 as is clear from the equations (3a) and (3b). Therefore, by obtaining data concerning the center coordinates (x.sub.v,y.sub.v) and an eccentricity K.sub.2 /K.sub.1 of the ellipse O.sub.3 from the detection signals x.sub.e and y.sub.e, preliminarily, a true moving direction .theta. can be easily obtained according to the following equation: EQU .theta.=tan.sup.-1 ((x.sub.e -x.sub.v)/( y.sub.e -y.sub.v)).multidot.(K.sub.2 /K.sub.1)) (5)
Japanese Patent Application Laid-open No. 24811/1983 discloses a typical example of a direction finder based on this principle. In this laid-open application, among the detection signals x and y obtained from the terrestrial magnetism sensor 2 when the vehicle 1 travels a circle route, maximum values x.sub.max and y.sub.max and minimum values x.sub.min and y.sub.min in the respective axes of the x-y perpendicular coordinates system are stored and the center coordinates (x.sub.o,y.sub.o) and x and y radii K.sub.x and K.sub.y of the ellipse O.sub.3 are obtained according to the following equations: EQU x.sub.o =(x.sub.max +xmin)/2 (6a) EQU y.sub.o =(y.sub.max +y.sub.min)/2 (6b) EQU K.sub.x =(x.sub.max -x.sub.min)/2 (6c) EQU K.sub.y =(y.sub.max -y.sub.min)/2 (6d)
Therefore, be performing the above operations at a suitable time and making K.sub.2 /K.sub.1 correspond to k.sub.y /K.sub.x and x.sub.v and y.sub.v to x.sub.o and y.sub.o, respectively, a true moving direction can be obtained according to the equation (5).
In the conventional direction finder, the ellipse locus is estimated by using the maximum values and minimum values of the respective detection signals obtained by an initial round movement of the vehicle as initial correction values and the moving direction of the vehicle is obtained by direction signals detected thereafter on the basis of the ellipse locus, as mentioned above. Therefore, a true moving direction can be obtained as long as the estimation conditions of the ellipse locus obtained by the initial correction do not change thereafter. However, since a magnetizing field H.sub.v of the vehicle may be changed due to vibrations within the vehicle and/or a variation of external magnetic field strength, thus resulting in errors in the initial estimation condition of the ellipse locus, it is usually difficult to obtain a true moving direction. This may be overcome by directing the vehicle around a circle route again each time an estimation condition is changed. However, as a practical matter it is very troublesome for an operator of a vehicle to perform such an operation every time such a change occurs.